Drug delivery systems are often critical for the safe and effective administration of a biologically active agent. Perhaps the importance of these systems is best realized when patient compliance and consistent dosing are taken under consideration. For instance, reducing the dosing requirement for a drug from four-times-a-day to a single dose per day would have significant value in terms of ensuring patient compliance and optimizing therapy. Many of the currently administered prodrugs and formulations require large amounts of drug/excipient/prodrug mixtures for administration.
Optimization of a drug's bioavailability has many potential benefits. For patient convenience and enhanced compliance it is generally recognized that less frequent dosing is desirable. By extending the period through which the drug is released, a longer duration of action per dose is expected. This will then lead to an overall improvement of dosing parameters such as taking a drug once a day where it has previously required four doses per day or dosing once a week or even less frequently when daily dosing was previously required. Many drugs are presently dosed once per day, but not all of these drugs have pharmacokinetic properties that are suitable for dosing intervals of exactly twenty-four hours. Extending the period through which these drugs are released would also be beneficial.
One of the fundamental considerations in drug therapy involves the relationship between blood levels and therapeutic activity. For most drugs, it is of primary importance that serum levels remain between a minimally effective concentration and a potentially toxic level. In pharmacokinetic terms, the peaks and troughs of a drug's blood levels ideally fit well within the therapeutic window of serum concentrations. For certain therapeutic agents, this window is so narrow that dosage formulation becomes critical.
In an attempt to address the need for improved bioavailability, several drug release modulation technologies have been developed. For example, poorly soluble 5,5-diphenylimidazolidine-2,4-diones have been derivatized into phosphate ester prodrugs to improve solubility (Stella et al., U.S. Pat. No. 4,260,769, 1981). Enteric coatings have been used as a protector of pharmaceuticals in the stomach and microencapsulating active agents using proteinaceous microspheres, liposomes or polysaccharides have been effective in abating enzymatic degradation of the active agent. Enzyme inhibiting adjuvants have also been used to prevent enzymatic degradation.
A wide range of pharmaceutical formulations provide sustained release through microencapsulation of the active agent in amides of dicarboxylic acids, modified amino acids or thermally condensed amino acids. Slow release rendering additives can also be intermixed with a large array of active agents in tablet formulations. Many of these formulations, however, deliver relatively low amounts of parent drugs in comparison with the overall weight of the formulations.
While microencapsulation and enteric coating technologies impart enhanced stability and time-release properties to active agent substances these technologies suffer from several shortcomings. Incorporation of the active agent is often dependent on diffusion into the microencapsulating matrix, which may not be quantitative and may complicate dosage reproducibility. In addition, encapsulated drugs rely on diffusion out of the matrix or degradation of the matrix, or both, which is highly dependent on the chemical properties and water solubility of the active agent. Conversely, water-soluble microspheres swell by an infinite degree and, unfortunately, may release the active agent in bursts with limited active agent available for sustained release. Furthermore, in some technologies, control of the degradation process required for active agent release is unreliable. For example, because an enterically coated active agent depends on pH to release the active agent and pH and residence time varies, the release rate is difficult to control.
Several implantable drug delivery systems have utilized polypeptide attachment to drugs. Additionally, other large polymeric carriers incorporating drugs into their matrices are used as implants for the gradual release of drugs. Yet another technology combines the advantages of covalent drug attachment with liposome formation where the active ingredient is attached to highly ordered lipid films.
There is a generally recognized need for sustained delivery of drugs that reduces the daily dosing requirement and allows for controlled and sustained release of the parent drug and also avoids irregularities of release and cumbersome formulations encountered with typical dissolution controlled sustained release methods. Furthermore, there is a need to accomplish the above listed goals with high relative ratios of parent drugs in relation to the dry weight of the formulation.